As the applications of wireless apparatus grow, the radiation patterns of antenna become more and more important in order to improve the communication quality. The antenna of conventional wireless apparatus is usually placed near the edge of the wireless apparatus to reduce the size of the whole system. Therefore, the ground plane is not symmetrical to the antenna. The radiation patterns of antenna are affected by the ground plane. When the antenna is placed near the edge of the wireless apparatus, the radiation angle θ with maximum radiation energy of antenna is normally located at θ>90°. This means that the direction of maximum radiation energy is inclined to the ground plane. The shift of maximum radiation direction due to the above mentioned unsymmetrical ground plane is towards the human body, and the human body can absorb the radiation energy. This absorption of the radiation energy then degrades the quality of communication. A novel design for controlling the direction of the radiation patterns of antenna is necessary to improve the quality of communication.
In US Patent Publication US 2004/0252056 A1 “U-Shaped Multi-Frequency Antenna of High-Efficiency”, a multi-frequency antenna design was disclosed. This kind of antennas has an angle θ of maximum radiation towards the lower half radiation plane (θ>90°). This angle of maximum radiation will lead to a significant absorption of the radiation energy by the human body. The transmitted signals on the front-end circuit may be interfered by the radiation energy as well. Furthermore, the radiation power on the horizontal plane is normally less than 0 dBi. This will lead to a poor quality of communication on the horizontal plane (θ=90°).
FIG. 1 is a conventional wireless apparatus structure with a monopole antenna element. The wireless apparatus 10 comprises a monopole antenna element 11, a ground plane 12, and an antenna feed-point 13. The distribution of current is shown in FIG. 2. The dashed line shows the magnitude of the current. A positive current 21 flows opposite to a negative current 22 and they have different magnitudes. This results in a shift of radiation patterns of antenna. FIG. 3 shows the radiation patterns of the wireless apparatus shown in FIG. 1. Referring to FIG. 3, the angle θ of maximum radiation of antenna is located at θ=130°. The power gain at θ=90° is less than 0 dBi. They are −7 dBi on the x-z plane and −5 dBi on the y-z plane, respectively. It is obvious that these kinds of radiation patterns and antenna gains shall affect the quality of communication.